1. Field of the Invention
The present invention relates to an image signal processing apparatus, and more particularly, to an image signal processing apparatus for generating a bright signal corresponding to chrominance signals by processing an image signal input from an analog-to-digital converter in a pixel-by-pixel basis.
2. Description of the Related Art
FIG. 1 illustrates an arrangement structure of color filters attached to charge-coupled devices (CCDs) in a digital camera that is a typical image signal processing apparatus. FIG. 2 illustrates a basic configuration of a typical image signal processing apparatus, e.g., a digital camera disclosed in U.S. Pat. Publication No. 2007-0174780. FIG. 3 illustrates a configuration of an image signal periodically output to a DB line from an analog-to-digital converter (ADC) illustrated in FIG. 2. FIG. 4 illustrates a configuration of an image signal periodically output to a DR line from the ADC illustrated in FIG. 2.
In FIG. 2, reference characters SB1, SB2, . . . , SBm denote signals input from the CCDs to the ADC via DB lines, reference characters SR1, SR2, . . . , SRm denote signals input from the CCDs to the ADC via DR lines, and reference characters DB1, DB2, . . . , DBm denote signals input from the ADC to the image signal processing apparatus via DB lines. Also, reference characters DR1, DR2, . . . , DRm denote signals input from the ADC to the image signal processing apparatus via DR lines, reference characters (R−Y)1, (B−Y)1, . . . , (R−Y)2m, (B−Y)2m denote chrominance signals output from the image signal processing apparatus, and reference characters YL1, . . . , YL2m denote chrominance signals output from the image signal processing apparatus.
In FIG. 1, Magenta (Mg), Cyan (Cy), Green (G), and Yellow (Ye) on the top are for DB lines, and G, Cy, Mg, and Ye on the bottom are for DR lines. A signal DBm output from the ADC to an mth DB line corresponds to Mg+Cy in an oddth horizontal period and G+Ye in an eventh horizontal period (referring to FIG. 3). Thus, in the image signal processing apparatus, a signal DBm(n) corresponding to an mth DB line in an nth horizontal period is finally obtained by using Equation 1.DBm(n)=Mg+Cy−(G+Ye)  (Equation 1)
In Equation 1, when n is an odd number, Mg+Cy denotes a signal applied to the mth DB line in the nth horizontal period, and G+Ye denotes a signal applied to the mth DB line in an (n+1)th horizontal period. When n is an even number, G+Ye denotes a signal applied to the mth DB line in the nth horizontal period, and Mg+Cy denotes a signal applied to the mth DB line in the (n+1)th horizontal period. For the reference, in terms of color, since Mg=R (Red)+B (Blue) and Cy=B+G, Equation 1 turns out 2B-G.
A signal DRm output from the ADC to an mth DR line corresponds to G+Cy in an oddth horizontal period and Mg+Ye in an eventh horizontal period (referring to FIG. 4). Thus, in the image signal processing apparatus, a signal DRm(n) corresponding to an mth DR line in an nth horizontal period is finally obtained by using Equation 2.DRm(n)=Mg+Ye−(G+Cy)=2R−G  (Equation 2)
In Equation 2, when n is an odd number, G+Cy denotes a signal applied to the mth DR line in the nth horizontal period, and Mg+Ye denotes a signal applied to the mth DR line in an (n+1)th horizontal period. When n is an even number, Mg+Ye denotes a signal applied to the mth DR line in the nth horizontal period, and G+Cy denotes a signal applied to the mth DR line in the (n+1)th horizontal period. For the reference, in terms of color, since Mg=R+B and Cy=B+G, Equation 2 turns out 2R-G.
The input image signals DB1, DB2, . . . , DBm are processed as described above, since pixel image signals DB1, DB2, . . . , DBm from the ADC overlap through two horizontal periods (sampling periods). Thus, if it is assumed that a horizontal frequency (a sampling frequency) is fs, as illustrated in FIG. 5, in an input image signal DB(n) or DR(n), an normal component of a bright signal YL(n) overlaps with a high frequency component that is a carrier frequency of a chrominance signal (R−Y)(n) or (B−Y)(n). The image signal processing apparatus uses a low pass filter (LPF) having characteristics of a first curve C1 illustrated in FIG. 6 in order to cancel a high frequency component of the chrominance signals (R−Y)(n) and (B−Y)(n) in a process of generating the bright signal YL(n) corresponding to the chrominance signals (R−Y)(n) and (B−Y)(n). In addition, the image signal processing apparatus uses an LPF having characteristics of a second curve C2 illustrated in FIG. 6 in order to cancel a high frequency component of a chrominance signal (R−Y)(n) or (B−Y)(n) in a process of obtaining the chrominance signal (R−Y)(n) or (B−Y)(n).
FIG. 7 illustrates an object image of a frame (FR) formed with the input image signals DB1, DR1, . . . , DBm, and DRm of the image signal processing apparatus illustrated in FIG. 2. Referring to FIG. 7, if it is assumed that only a single object exists, a range of the object is determined by a horizontal width WH and a vertical width WV.
As described above, saturation is relatively high in an object area not a background area. Accordingly, a normal high frequency component exists in a bright signal.
Nonetheless, according to a conventional image signal processing apparatus, in order to cancel a high frequency component of chrominance signals in a process of generating a bright signal, all of input image signals DB1, DR1, . . . , DBm, and DRm pass through an LPF (referring to C1 of FIG. 6). That is, low pass filtering is performed for all areas of a frame FR in the process of generating a bright signal.
Accordingly, since a high frequency component of the bright signal existing in an object area is canceled in the process of generating the bright signal, image reproducibility is degraded.